Add opcode to check for returning `None` in `__init__`

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python/cpython@main...faster-cpython:cpython:specialize-calls-to-python-classes

This is "easy" for normal def __init__ but harder for __init__ = x and ty.__init__ = x because you have to hook and re-instrument or something.

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Unless---oops---it actually puts that opcode after CALL_FUNCTION to type, instead of inside the method

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Oh it's a shim frame, neat

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I wonder how we could push that frame very fast without doing the sort of static-alloc and pointer link thing they do in CPython

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python/cpython@04492cb